As for conventional active materials of positive electrodes used for alkaline secondary batteries such as nickel-cadmium secondary battery, nickel-hydrogen secondary battery, nickel-zinc secondary battery, nickel-iron secondary battery and others, there are known active materials of positive electrodes which are made by adding and mixing at least one kind of such cobalt compounds as metallic cobalt powder, cobalt hydroxide or cobalt monoxide etc. as an additive to increase electric conductivity and utilization rate of the active material, with nickel hydroxide powder which is an active material or a nickel hydroxide powder with any metallic element such as cobalt, zinc, etc. formed in the solid solution. This active material of the positive electrode is mixed and kneaded with an aqueous solution of a thickener such as carboxymethyl cellulose etc. to form a paste, and then the paste is filled in an alkaline-resistant porous current collecting substrate made of a foamed nickel or a felt-like nickel etc., and then is dried and pressed to produce a paste type nickel electrode. The paste-type nickel electrode used as the positive electrode is stacked with a negative electrode one upon another with a separator interposed therebetween to form a plate group. The plate group is put in a battery container and a given amount of alkaline electrolyte is poured therein. Thereafter, a cover is applied thereto and an alkaline secondary battery is produced.
The above-mentioned metallic cobalt or cobalt compound reacts with hydroxide ions in the electrolyte and water soluble cobalt complex ions are formed and diffused, and are oxidized by charging of the battery, so that the conductive cobalt oxyhydroxide CoOOH for coating the surface of the nickel hydroxide is precipitated to form the so called electric conductive matrix network, so that electric conductivity of the active material of the paste-type nickel electrode, that is, between the nickel hydroxide particles is improved, and accordingly the intention of the improvement in utilization rate of the active material is achieved.
Further, as a prior art, JP-A 92-234867 discloses a method of producing an active material of a positive electrode. In the method, nickel hydroxide powder is immersed in an aqueous solution of metallic cobalt or cobalt salts such as cobalt sulfate, cobalt acetate, etc., added with sodium hydroxide to be made to react therewith in a weak alkaline region. Then, .alpha.-Co(OH).sub.2 or .beta.-Co(OH).sub.2 is precipitated, and there is obtained an active material for the nickel positive electrode in which the surface of the nickel hydroxide is coated with any of these cobalt compounds. A nickel electrode used for an alkaline battery is prepared by filling this material in a porous collecting substrate.
Furthermore, in JP-A 8-236108, there is disclosed a method of producing a nickel positive electrode in which a nickel hydroxide is produced. An aqueous solution of a cobalt salt and an aqueous solution of an alkali metallic hydroxide such as sodium hydroxide etc. are added to the slurry of nickel hydroxide and stirred to be made to react one with another while maintaining the alkaline condition of pH 11.5.+-.0.3 to produce an active material in which a layer of cobalt hydroxide is formed on the surfaces of nickel hydroxide particles. A paste thereof is added and kneaded with such an additive as a metallic cobalt powder or the like to obtain an active material paste, which is filled in the porous substrate.
Furthermore, in JP-A 8-227712, it is disclosed that a solution of cobalt salt and an aqueous alkali solution are reacted in the alkaline region under the presence of a reducing agent such as hydrozine to precipitate cobalt hydroxide Co(OH).sub.2. At the same time, the oxidation reaction of cobalt hydroxide caused by dissolved oxygen in the reacting solution is inhibited due to the presence of the reducing agent. As a result, formation of an inactive higher-order cobalt oxide such as Co.sub.3 O.sub.4 on the surface of Co(OH).sub.2 is prevented, and Co(OH).sub.2 powder which is obtained by being precipitated, is washed with water and dried, and Co(OH).sub.2 thus obtained is then added to nickel hydroxide and the paste-type active material thereof is filled in the porous electrode substrate to produce a nickel positive electrode.
After the paste-type nickel electrode filled with the mixture of the nickel hydroxide powder added with the powder of cobalt compounds such as cobalt monoxide or cobalt hydroxide etc. is incorporated in an alkaline battery, the cobalt compound is converted into cobalt oxyhydroxide in the paste-type nickel electrode by charging. This method of forming the electric conductive network has the following inconveniences. That is to say, since the cobalt compound itself, which is mixed among the nickel hydroxide particles, has no electric conductivity, the polarization at the initial charging is increased. It is feared a bad influence by generating gases, and accordingly there are involved such inconveniences that the charging has to be carried out with a small current. To ensure a rated charge capacity, a long charging time is required. Also in the case of metallic cobalt, it has such an inconvenience that because the surface thereof is oxidized to become a coat of cobalt oxide, it is unavoidable to require a long time for the initial charging as the cobalt oxide does.
As there is disclosed in JP-A62-234867, particles of cobalt hydroxide Co(OH).sub.2 precipitated by reacting the nickel hydroxide particles with cobalt salt and alkaline solution in an alkaline region of the reaction solution before filling an active material of nickel hydroxide in the porous collective substrate, are obtained as .alpha.-Co(OH).sub.2 which is high in crystallizability, as it is clear from the X-ray diffraction patterns shown in FIG. 1 and FIG. 2, and therefore the surface area thereof is small, and accordingly reactivity thereof with alkaline electrolyte is low. Also .beta.-Co(OH).sub.2 is easily oxidizable and the crystal grains thereof are easily turned into higher-order oxides such as inactive tricobalt tetroxide or the like with the lapse of time so that the electric conductivity thereof is lowered. Therefore, when the paste-type nickel electrode is produced by using the positive electrode active material prepared by mixing the particles thereof in cobalt hydroxide particles, there is caused such a problem that the utilization rate of the active material is lowered. Also the active material of a positive electrode has such an inconvenience that the oxidization thereof is progressed during preservation, and the preservation and control thereof become difficult.
Furthermore, a method of producing the active material of the positive electrode disclosed in JP-A 8-236108 is also such that the cobalt hydroxide is precipitated in an alkaline region. Accordingly the particles of cobalt hydroxide have the same high-crystallizability as the cobalt hydroxide in the invention disclosed in JP-A8-227712, so that it is easily oxidizable by contacting it with dissolved oxygen in the reaction solution or oxygen in the air, and easily becomes the inactive high-order oxides. Accordingly this involves such inconvenient problems that the utilization rate of the paste-type nickel electrode using this material is lowered and that the preservation and control of the positive active material thus produced become difficult.
Furthermore, according to the invention disclosed in JP-A 8-27712, in the case where the cobalt salt solution and the aqueous alkali solution are made to react one with another under the presence of the reducing agent, the oxidization of precipitated cobalt hydroxide caused by oxygen dissolved in the reacting solution can be avoided. However, if the reducing agent remains in the nickel hydroxide even by washing with water after completion of the reaction, it causes such problems as accelerating the self-discharge of the battery and the like. In addition, if washing with water is carried out completely and the reducing agent is removed completely, it is susceptible to be oxidized, and at the same time, inactive higher-order oxides are formed on the surface thereof and the electric conductivity of the electric conductive matrix is lowered.
Accordingly, it is desired to solve the above-mentioned conventional problems to produce cobalt hydroxide which is difficult to be oxidized and thereby to obtain such an active material for a positive electrode used for alkaline second batteries that is good in its reaction with an electrolyte and forms an electric conductive matrix which has a good electric conductivity in the paste-type nickel electrode.